Optical dispersion, that is, the separation of light into its constituent wavelength components, is a phenomenon used by a wide variety of applications, including Raman and fluorescence detection and other forms of spectral analysis. In addition, the emerging field of optical communications uses optical dispersion to perform wavelength multiplexing and demultiplexing, filtering and other functions. Although the concept of optical dispersion has been known for quite some time, the earliest apparatus utilized prisms as a diffraction means. Optical gratings were later developed for this purpose, and, since the invention of holography, holographic gratings have been applied to this task with enhanced efficacy.
It is known to pass polychromatic light through a pair of identical gratings that act together to provide an output beam which is both collimated and laterally dispersed. Such an arrangement is disclosed by E. B. Treacy in “Optical Pulse Compression With Diffraction Gratings,” IEEE Journal of Quantum Electronics, Vol. QE-5, No. 9, September, 1969, which finds particular application in pulse compression for ultrafast laser systems that employ chirped-pulse amplification. The first grating diffracts each wavelength through a different angle according to the grating equation, thereby introducing angular dispersion to the polychromatic beam, so that the beam spreads as it propagates from the first grating toward the second. The second grating diffracts each wavelength again through the same angle, but in the opposite direction, so that the beam leaves the second grating in the same direction as the beam that was incident to the first grating, with the various wavelengths being spread laterally but propagating in exactly the same direction, or recollimated. One disadvantage of this configuration is that the gratings and auxiliary optics are separate elements that must be individually mounted and aligned, with the attendant risk of alignment drift with time or mechanical motions such as vibration.
It is also known that dispersion may be increased by passing light through a plurality of gratings, each grating further dispersing the light incident to it. In “Double dispersion from dichromated gelatin volume transmission gratings,” Proceedings of the SPIE, Vol. 1461, 1991, D. E. Sheat, G. R. Chamberlin, and D. J. McCartney disclosed a configuration wherein light is passed through a single grating two times with the aid of a mirror, either separate from the grating or made part of the grating to form an integrated device. However, this configuration is limited to two passes of the light through the grating, and the beam that exits from the integrated device is counter-propagating with respect to the incident beam, so that separating the input and output beams requires additional optical components or performance compromises. Moreover, the configuration described by Sheat, et al. only produces angularly dispersed light, so that conversion to a laterally dispersed, collimated beam again requires additional optical components.
There are also described in the literature dispersive optical elements specifically intended for optical communications. Such a structure is described by Y. Huang, D. Su, and Y Tsai in “Wavelength-division-multiplexing and—demultiplexing by using a substrate-mode grating pair,” Optics Letters, Vol. 17, No. 22, Nov. 15, 1992. According to this device, within a substrate-mode element there are two distinct gratings which first angularly disperse and then recollimate incident light. The output channel separation or the spatial dispersion of such a structure is directly related to the angular dispersion obtained through the first grating and the distance the dispersed light travels before being collimated by the second grating. The amount of dispersion in the substrate-mode element is therefore dependent on the length as well as the thickness of the substrate. In a practical sense, the substrate must therefore be long to provide substantial optical distance between the dispersing grating and the collimating grating to obtain high degree of spatial dispersion. Additionally, the space between the dispersing and collimating grating cannot include a grating, or the total internal reflection necessary for propagation would be prevented.
Another prior-art device is described by R. Kostuk, et. al. in “Reducing alignment and chromatic sensitivity of holographic optical interconnects with substrate-mode holograms,” Applied Optics, Vol. 28, No. 22, Nov. 15, 1989. The structure of the substrate-mode element described in this paper incorporates a holographic grating as an input element to produce a+1 and a+1 diffracted order from the incident light. These orders propagate through the substrate by means of multiple internal reflections until intercepted by holographic optical elements which redirect, focus, and couple each beam out of the structure and onto receivers. The purpose of this structure is to produce multiple beams output into some preferred spatial arrangement from a single incident beam of coherent light.
The term “grism” refers to a grating-prism combination. More technically, a grism (or Carpenter's prism) is formed by replicating a transmission grating onto the hypotenuse face of a right-angle prism. The spectrum produced by the grating is deflected by the prism to remain on the optical axis and the apex angle of the prism is chosen to get a certain wavelength in the center of the detector. Grisms are useful in spectrometers that require in-line presentation of the spectrum. The light diffracted by the grating is bent back in line by the refracting effect of the prism. A typical use for grisms is to unfold the light path in that the entrance prism and the exit prism apex angles compensate for the input and exit angles of light to and away from the grating resulting in a non-displaced central wavelength. Another typical use is to provide an entrance and exit window for light to enter and exit the grating which otherwise would exceed the critical angle.
U.S. Pat. No. 5,530,565 describes a narrow bandwidth bandpass filter having high transmission efficiency for the passband and excellent out-of-band attenuation employs a transmission holographic grating sandwiched between the oblique faces of a pair of right angle glass prisms. An incoming laser beam to be filtered is incident normal to one of the prism faces so as to intersect the holographic grating at about 45 degrees. The grating frequency is such as to diffract light of the transmission wavelength through substantially 90 degrees so that it exits the cube formed by the two prisms from the right angle face of the second prism. The out-of-band wavelengths of the incident beam are either transmitted unaffected through the grating or diffracted at a different angle than the light of the transmission wavelength. A spatial filter comprising a mask with a central aperture is supported a spaced distance from the output face of the cube so that diffracted light of the transmission wavelength passes through the aperture and the unwanted wavelengths, which are diffracted at different angles than the transmission wavelength, are blocked by the mask.
U.S. Pat. No. 6,278,534 is directed to a compact, preferably monolithic optical element converts an incident beam of light into a dispersed exit beam. A transmissive optical grating is supported between two reflective surfaces such that a beam is reflected to pass through the same grating at least twice to form the exit beam. In the preferred embodiment, the grating is a volume hologram cemented between two optically transmissive substrates which include outwardly oriented surfaces that are parallel to one another and to the grating, and the internal reflections occur at these surfaces. Mirrors may also be used. A preferred method of grating formation is also disclosed.
In U.S. Pat. No. 6,449,066, a volume-phase optical grating, preferably supported between substrates and prisms, uses large-angle input and output light beams to provide a very high degree of dispersion and improved separation of closely spaced wavelength channels. The average refractive index of the grating medium is also less than that of the supporting substrates and prisms, thereby providing improved uniformity and reduced sensitivity to the state of light polarization. The device therefore finds utility as a wavelength multiplexer, demultiplexer or optical spectrum analyzer in fields such as optical communications and optical signal processing. The grating itself may be constructed by conventional interferometric or holographic techniques, and may be a reflection or transmission device. In a system configuration, optical fibers may be used to carry the multiplexed or demultiplexed optical signals. Optoelectric detectors may also be used to detect different wavelengths and convert the optical signals into electrical counterparts. Alternatively, electrical signals may be converted to optical signals of differing wavelength, and these may be multiplexed using one or more of the inventive devices.
Despite these advances, there continues to exist an outstanding need for an optically dispersive structure which may take advantage of the same grating to achieve a multiplicative dispersive effect, ideally, to achieve a high degree of direct lateral dispersion from a monolithic component.